Although it will become evident to those skilled in the art that the present invention is applicable to a variety of implantable medical devices utilizing pulse generators to stimulate selected body tissue, the invention and its background will be described principally in the context of a specific example of such devices, namely, cardiac pacemakers for providing precisely controlled stimulation pulses to the heart. The appended claims are not intended to be limited, however, to any specific example or embodiment described herein.
Pacemaker leads form the electrical connection between the cardiac pacemaker pulse generator and the heart tissue which is to be stimulated. For example, endocardial type leads, that is, leads which are inserted into a vein and guided therethrough into a cavity of the heart, include at their distal end an electrode head having a tip designed to contact the endocardium, the tissue lining the inside of the heart. As is well known, the leads connecting such pacemakers with the heart may be used for pacing; or for sensing electrical signals produced by the heart; or for both pacing and sensing in which case a single lead serves as a bidirectional pulse transmission link between the pacemaker and the heart.
One type of existing implantable pacing lead assembly includes an electrode head having an activated vitreous carbon tip forming the conductive region or stimulation surface maintained in contact with the heart tissue to be stimulated. As is well known, vitreous carbon electrode tips, as well as other electrode tip materials in present use, have several advantages. They can provide relatively low chronic pacing thresholds, improved sensing characteristics and lower polarization voltages. On the other hand, existing electrode tips are relatively rigid, that is, they are substantially less compliant than the heart tissue with which they are in contact. Accordingly, such electrode tips subject the heart tissue to relatively high levels of mechanical stress or pressure concentration which, because of relative movement between the compliant tissue of the beating heart and the noncompliant electrode tip, results in trauma and inflammatory tissue reaction. Among other undesirable consequences, this adverse reaction results in an increase in the stimulation threshold of the heart.
Thus, it is one object of the present invention to provide an implantable medical device lead assembly having an electrode head with structural characteristics minimizing the induced stress on the endocardium and therefore the adverse reaction by the heart to the presence of the electrode head.
To prevent displacement or dislodgement of the electrode head and to maintain the necessary stable electrical contact between the lead tip and the endocardial tissue, the electrode head must be firmly anchored. To achieve this, the electrode head of existing leads includes pliant, rearwardly projecting anchoring means, such as fins or tines, for secure positioning of the electrode head. These fixation elements become anchored in the heart's trabecular network and reduce the incidence of electrode head displacement or dislodgement. In this fashion, the position of the electrode tip is mechanically stabilized, that is, the tip is positively anchored so as to remain in place during the lifetime of the implant. However, although the flexible fixation elements tend to fold down against the outer surface of the lead assembly during its insertion in the vein, these elements nevertheless continue to project to some extent thereby causing resistance to the advancement of the lead assembly within the vein.
Accordingly, another object of the present invention is to reduce the frontal area of the electrode head during implantation and thereby reduce the resistance to advancement of the lead within the vein.